i would say ethanol (grain alcohol) bro. Mine have stayed in solution for months now in ethanol. Just go to liquor store and pick up some everclear. This is what i would do if i were you:
What you would need:
1) Letrozole powder (1,000mg)
2) (4) 100mL vials
3) A measuring cup that can hold atleast 500mL
4) 30cc syringe or bigger, to minimize time (i prefer 60cc Terumo syringe, doesnt take as long)
5) accurate scale
6) 400mL Everclear
Say you wanted to make the solutions 2.5mg/mL. You have 1g (1000mg) of letrozole. so that equals 400mL final product. Remember, 1g usually displaces 1mL of liquid, so you will need 399mL of everclear.
First, measure out 399mL of everclear with the syringe, and put it in the measureing cup that holds 500mL or more (now you know why i said a 30cc syringe or bigger
)
Second, measure out 1,000mg of letrozole powder, and add it to the everclear.
Third, stir it up a little bit and heat up.
Fourth, fill up the 100mL vials.
There is no need to filter it, as you will be using it orally, so sterility is not of concern. Now wasnt that easy?